1. Field of the Invention
The present invention relates to an insulated gate GTO thyristor.
2. Description of the Related Art
FIG. 16 is a sectional view showing a conventional insulated gate GTO thyristor. This structure is the same as that in FIG. 4 of U.S. Pat. No. 4,604,638. This insulated gate GTO thyristor has a pnpn structure constituted by a p.sup.+ -type emitter layer 1, an n.sup.+ -type buffer layer 2, an n-type base layer 3, a p-type base layer 4, and an n.sup.+ -type emitter layer 5. An anode electrode 7 is formed on the surface of the p.sup.+ -type emitter layer 1. A cathode electrode 8 is formed on the surface of the n.sup.+ -type emitter layer 5. A p.sup.+ -type layer 6 is formed in the p-type base layer 4, and a first gate electrode 9 is formed on the surface of the p.sup.+ -type layer 6. A second gate electrode 11 is provided on the surface of the p-type base layer 4 between the n-type base layer 3 and the n.sup.+ -type emitter layer 5 to be insulated by a gate insulating film 10.
A turn-ON operation of the insulated gate GTO thyristor will be described below. A positive voltage with respect to the cathode electrode 8 is applied to the first gate electrode 9 and the second gate electrode 11 when the thyristor is to be turned on. At this time, an inversion layer is formed in the surface portion of the p-type base layer 4 immediately below the gate insulating film 10 to form a channel, and electrons are injected from the n.sup.+ -type emitter layer 5 into the n-type base layer 3 through the channel. When the electrons injected in the n-type base layer 3 reach the p.sup.+ -type emitter layer 1, holes are injected from the p.sup.+ -type emitter layer 1 to the n-type base layer 3. The holes are collected in the n.sup.+ -type emitter layer 5 through the p-type base layer 4. As a result, electrons are directly injected from the n.sup.+ -type emitter layer 5 to the p-type base layer 4, thereby turning on the element.
When the thyristor is to be turned off, a negative voltage with respect to the cathode electrode 8 is applied to the first gate electrode 9 and the second gate electrode 11. In this case, since the channels are lost and a reverse bias voltage is applied to a p-n junction between the n.sup.+ -type emitter layer 5 and the p-type base layer 4, holes stored in the p-type and n-type base layers 3 and 4 are discharged to the base electrode 9, thereby turning off the element.
In order to improve the turn-OFF ability of the insulated gate GTO thyristor described above, an amount of base current extracted from the first gate electrode 9 in a turn-OFF operation must be increased. As one of methods of increasing the amount of base current, assuming that the sheet resistivity of the p-type base layer 4 is set to be .rho.s and the breakdown voltage of the emitter junction formed by the n-type emitter layer 5 and the p-type base layer 4 is set to be Vj, the value of Vj/.rho.s is increased.
The breakdown voltage Vj of the emitter junction is determined by the minimum breakdown voltage of three breakdown voltages at points indicated by symbols a, b, and c. In these breakdown voltages, the breakdown voltage at the point a is determined by a concentration distribution on the surface of the p-type base layer 4 of the channel portion and a concentration distribution on the surface of the n.sup.+ -type emitter layer 5. In order to increase these breakdown voltages, basically, concentrations of the n.sup.+ -type emitter layer 5 and the p-type base layer 4 are decreased. However, the n.sup.+ -type emitter layer 5 must have a high concentration to serve as an emitter, and the n.sup.+ -type emitter layer 5 cannot have a low concentration.
On the other hand, since the p-type base layer 4 serves as a channel at the point a, the concentration of the p-type base layer 4 must be kept constant to hold an optimal threshold value. For this reason, the concentration of the p-type base layer 4 cannot be arbitrarily changed. In addition, as the concentration of the p-type base layer 4 is increased, its sheet resistivity .rho.s is decreased. Therefore, the p-type base layer 4 preferably has a high concentration to improve the turn-OFF ability. However, its concentration cannot be changed due to the above-described reason.
As described above, in the insulated gate GTO thyristor the concentrations of the n.sup.+ -type emitter layer 5 and the p-type base layer 4 cannot be arbitrarily set. Therefore, the value of Vj/.rho.s cannot be increased, and a high turn-OFF ability is difficult to obtain.